I'm a set collector first and foremost. I left the Hobby on 01 and sold everything off, but came back in 207 starting with nothing in hand. Funnily enough, I started with 74 also (birth year), but have also started moving backwards in time. Part of what you'll run into is pandemic related, especially in vintage. I went backwards as I felt even some of the key 60s cards (Ryan RC, Rose RC, etc.) were obtainable with some patience and timing. Then the pandemic came and many of the key cards are in high demand and/or in the High Series as well. Sigh.... As far as your questions
1 - I get almost all my vintage from shows and not eBay. With the handful of dealers I have gotten great deals on vintage that you'll never get on eBay. Benenfit is you also get to see exactly what you are getting. Pays to have that personal relationship. Specific to High Series vs key cards, it all depends what is available. I've passed on HOF IOT to get some high series as those are much harder to come by. It all depends on what is in front of me and what my budget says I can do at that particular time too.
2 - Coming back in I bought everything as sets (74 onward). I've had to upgrade my 70s sets as I got them off an unscrupulous former member of this site. I bought a whoile bunch of FS and had the same bricking issues from 94-04. 94 and 01 I had to repurchase as hand collated due to the bricking, but I was able to rescue most of the others with minimal upgrading requirements. For me, it was easier and more cost efficient that building from scratch (no matter how much I love ripping wax). Mind you, this was all pre-pandemic pricing stupidity. But if you do hand build, for flagship sets from 1995 onwards it shouldn't be too difficult to find singles if you are staying with Topps. The others (Upper Deck, Donruss, etc) may be a little more difficult as they had years where they shortprinted many base cards. Heck, that was one of the reasons I got in in 2001 to begin with.
Tired and trembling
I am descending,
will I have to stay here
and live this life again?